1. Field of the Invention
The present invention relates to a beam position adjusting method for an optical scanning device used in, e.g. an electrophotographic digital copying machine, an image forming apparatus having the optical scanning device, and an image forming method.
2. Description of the Related Art
In the technical field of image forming apparatuses such as high-speed digital copying machines and high-speed printers, there is known an optical scanning device using, e.g. a multi-beam exposure device which simultaneously scans two or more beams and exposes a charged area on a photosensitive drum.
The multi-beam exposure device comprises, for example, a plurality of semiconductor laser elements for emitting laser beams; an optical member for adjusting the cross-sectional beam dimensions of the laser beams emitted from the semiconductor laser elements in accordance with a required resolution and setting an inter-beam distance between the laser beams in a sub-scan direction (a direction perpendicular to the axis of the photosensitive drum); a deflection device for deflecting and scanning the laser beams together in a main scan direction (an axial direction of the photosensitive drum); and a focusing lens system for guiding and focusing the laser beams deflected by the deflection device on a charged area on the photosensitive drum.
A galvano-mirror is used as an optical member used in the multi-beam exposure device. The galvano-mirror can minutely alter the angle of the mirror surface by controlling current supplied to an electromagnetic coil.
For example, in a multi-beam exposure device that simultaneously scans two laser beams, that is, a stationary laser beam and a movable laser beam, the angle of the mirror surface of the galvano-mirror is finely adjusted to control the distance (or the position) of the movable laser beam in the sub-scan direction.
The adjustment of the distance is effected by using a sensor such as a photodiode. Specifically, a laser beam is caused to scan the light-receiving surface of the sensor and a position on the light-receiving surface, at which the laser beam passes, is detected. This operation is performed for each laser beam. Thus, the distance between the laser beams is detected, and is adjusted based on the detection result.
When the inter-beam distance is to be adjusted, a circuit offset has to be considered. In other words, an output from the sensor is converted to a voltage signal by a current/voltage conversion circuit, and the voltage signal is amplified by an amplifier circuit and finally output via an integration circuit, etc. Therefore, a circuit offset in these circuits needs to be considered.
A description will now be given of a case where a distance (or a position) of a movable laser beam in the sub-scan direction is set relative to a stationary laser beam.
In the prior art, in the state in which a stationary laser beam is incident on a certain position on the light-receiving surface of the sensor, a galvano-mirror is used to effect a control to position a movable beam within a tolerable range of a beam position A that is a target position. Then, the movable laser beam is turned off, and a circuit offset relating to the beam position A is measured (S1).
Then, the galvano-mirror is used to effect a control to position the movable laser beam at a beam position B that is a target position. Then, the movable laser beam is turned off, and a circuit offset relating to the beam position B is measured (S2).
Subsequently, the galvano-mirror is used to effect a control to exactly position the movable laser beam once again at the beam position A by subtracting the circuit offset measured in step S1. An output value DA of the sensor at that time is read and stored in a memory.
Following the above, the galvano-mirror is used to effect a control to exactly position the movable laser beam once again at the beam position B by subtracting the circuit offset measured in step S2. An output value DB of the sensor at that time is read and stored in the memory.
The distance between the beam position A and beam position B is determined according to a resolution to be set. For example, if the resolution is 600 dpi, the distance is 42.3 μm.
The galvano-mirror is controlled to set the beam position of the movable laser beam such that a difference (DA-DB) between the obtained output values DA and DB of the sensor may become equal to a difference (DS-DM) between the output value DS obtained from the sensor when the stationary laser beam is incident on the sensor and the output value DM obtained from the sensor when the movable laser beam is incident on the sensor.
Thereby, the distance between the stationary laser beam and movable laser beam is set at a value corresponding to a predetermined resolution.
In the above-described prior-art beam position adjustment, however, the following controls are required:    (1) The control to position the movable laser beam within the tolerable range of the beam position A that is the target position, by using the galvano-mirror, thereby measuring the circuit offset at the beam position A.    (2) The control to position the movable laser beam within the tolerable range of the beam position B that is the target position, by using the galvano-mirror, thereby measuring the circuit offset at the beam position B.    (3) The control to position the movable laser beam at the beam position A with the circuit offset being taken into account, by controlling the galvano-mirror.    (4) The control to position the movable laser beam at the beam position B with the circuit offset being taken into account, by controlling the galvano-mirror.
The above controls (1) to (4) are effected in succession, thus adjusting the distance between the stationary laser beam and movable laser beam in accordance with the resolution. In other words, the position of the movable laser beam has to be varied from the beam position A, to the beam position B, to the beam position A, to the beam position B, and to the ultimate adjusted position, while operating the galvano-mirror which is very difficult to adjust. Thus, the adjustment of the laser beam position is time-consuming.
The positional adjustment of the laser beam is performed at the time of warm-up after power-on, completion of elimination of jamming, completion of pre-heating, etc., in the case of digital copying machines. However, since the adjustment of the laser beam position is time-consuming, a great deal of time is required until the start of the copying of a first sheet.